1. Field of the Invention
The invention relates to the field of thermal spraying of coatings onto objects and in particular the thermal spraying of nanocrystalline materials.
2. Description of the Prior Art
Significant interest has been generated recently in the field of nanoscale (nanocrystalline, nanophase ) materials. This arises from the outstanding properties that can be obtained in such materials. Also there is a realization that early skepticism about the ability to produce high quality, unagglomerated nanoscale powder was unfounded.
The present focus is shifting from synthesis to processing, i.e., to the manufacture of useful coatings and structures from these powders. The potential applications span the whole spectrum of technology, from thermal barrier coatings for turbine blades to wear resistant rotating parts. The potential economic impact is several billions of dollars per year.
Significant progress has been made in various aspects of processing on nanoscale materials. Most of this work has been focused on the fabrication of bulk structures See E. Y. Gutmanas, L. I. Trusov, and I. Gotman, NanoStructured Matls., 8 (1994) 893-901; G. E. Fougere, L. Riester, M. Ferber, J. R. Weertman, and R. W. Siegel, Mat. Sci. Eng., A204 (1995) 1-6; and G. E. Korth and R. L. Williamson, Metallurgical and Materials Transactions A, 26A (1995) 2571-1578.
A great deal of effort has gone into enhancing our understanding of the synthesis and structural characteristics of nanocrystals. More recently, greater scientific emphasis is being placed on the physical and mechanical characteristics of nanocrystalline ceramics and metals, since it is evident that it is possible to achieve combinations of properties that are otherwise unachievable with equilibrium materials.
For example, it is possible to sinter nanophase ceramics at temperatures that are substantially lower than those required by coarse grained ceramics, due to their fine microstructures, small diffusion scales, and high grain boundary purity. Nanophase ceramics are reported to exhibit unusually high ductility, whereas nanophase metals are noted to exhibit ultra-high hardness values.
In addition, recent work suggests improvements in other physical properties. For example, it has been shown that the thermal expansion coefficient of a nanocrystalline Ni-P alloy (21.6.times.10.sup.-6 K.sup.-1) is 56% higher than that of the coarse grained material of equivalent composition. It has been suggested that since the specific heat of a material is intimately related its vibrational and configurational entropy, the observed behavior may be attributable to the complicated structure associated with the grain boundaries of the nanocrystalline material.
In related work it was demonstrated that it was possible to obtain a high saturation flux density, a low magnetostriction, and excellent soft magnetic characteristics in nanocrystalline Fe-B-M materials where M=Cu, Nb, Mo, W, Ta.
What is perhaps most unusual about nanocrystalline materials is the fact that, despite being classified as nonequilibrium materials, recent work shows that their grain size may, in some cases, remain metastable during exposure to elevated temperatures. Although this phenomenon is not clearly understood, it has been suggested that the unusual resistance of the nanocrystals to coarsening may be due to their narrow size distribution.
A wide range of preparation methods have been developed for the fabrication of nanocrystalline materials. See, C. Suryanarayana, International Materials Reviews, 40 (1995) 41-64. However, these are largely regarded as a two step processes in which nanocrystalline material is first synthesized in powder form and subsequently consolidated into bulk form.